Non-aqueous electrolyte rechargeable batteries used as a main power source of mobile telecommunication equipment and mobile electronic devices of recent model feature high electromotive force and high energy density. Known examples of positive electrode active materials used therein are composite oxides such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2). These composite oxides have a potential of 4 V or higher with respect to metallic lithium.
Each of the composite oxides used as the positive electrode active material has advantages and drawbacks. Accordingly, an attempt has been made to use two or more composite oxides in mixture. For example, it has been proposed to use a spinel-type lithium-manganese composite oxide containing a heteroelement in mixture with a spinel-type lithium-manganese composite oxide containing a heteroelement and cobalt (see Japanese Laid-Open Patent Publication No. 2000-315503, claims 1 and 11 and page 4). This publication has also proposed to make the positive electrode active material include a third oxide having favorable electron conductivity.
Further, it has also been proposed to mix a lithium-cobalt composite oxide containing a heteroelement and a lithium-nickel composite oxide (see Japanese Laid-Open Patent Publication No. 2002-319398, claim 1 and page 3).
Among the composite oxides described above, the spinel-type lithium-manganese composite oxide and lithium-nickel composite oxide containing Mn are excellent in safety under high potential, cycle characteristics and heat resistance during overcharge.
According to Japanese Laid-Open Patent Publication No. 2000-315503, the positive electrode active material contains, as main components, both of the spinel-type lithium-manganese composite oxide whose crystal expands due to intercalation of lithium ions and the lithium-nickel composite oxide whose crystal shrinks due to intercalation of lithium ions. Therefore, due to an interaction between them, contact among active material particles with progress of cycles is maintained in stable condition, thereby inhibiting reduction in cycle characteristics.
Accordingly, there arises a problem in that the advantages of the composite oxides are averaged, making their peculiarities indistinct. That is, superiority cannot be gained over a battery using a spinel-type lithium-manganese composite oxide featuring low costs and high safety as a main active material or a battery using a lithium-nickel composite oxide aiming at high capacity as a main active material.
Further, in Japanese Laid-Open Patent Publication No. 2002-319398, the same problem arises when a lithium-cobalt composite oxide containing a heteroelement is mixed with a lithium-nickel composite oxide having high capacity, because the capacity of the lithium-cobalt composite oxide is low. In particular, if the heteroelement contained in the lithium-cobalt composite oxide is other element than magnesium, capacity reduction occurs remarkably.
Still further, there arises another problem when the spinel-type lithium-manganese composite oxide or lithium-nickel composite oxide containing Mn is used as the positive electrode active material. That is, manganese in high potential state at high temperature is apt to be resolved from the positive electrode to an electrolyte. Thus, these oxides have problems in cycle characteristics and shelf life at high temperature.